As electrical energy storage devices utilizing a capacitor with large capacitance, electrolytic capacitors and multi-layer ceramic capacitors are generally used. In recent years, with the increasing demand for batteries of large capacity typically in electric vehicles, the increased capacity of lithium-ion batteries has been promoted as well as electric double-layer batteries having the function of electrolytic capacitor enhanced have actively been developed. In addition, a research for increasing the capacitance of thin-film type of multi-layer ceramic capacitors has started.
However, so-called capacitors, such as a multi-layer ceramic capacitor, involve a problem with electric current leakage, which deteriorates the general performance. Further, so-called batteries storing electrical energy by means of chemical change, such as a lithium-ion battery, involve a problem of memory effect caused, when partially being charged and discharged, and resulting in deteriorating the performance.
In view of solving the problems, for example an electrical energy storage device using a giant magneto-resistance effect is proposed in Patent Documents 1, 2 and 3.
The giant magneto-resistance effect (GMR) means a kind of quantum-mechanical effect observed in a mechanism formed alternately by a thin magnetic section and a thin nonmagnetic section. When an external magnetic field is applied, the giant magneto-resistance effect causes the electric resistance to largely change from the state of high resistance with a zero magnetic field to the state of low resistance with a high magnetic field. Therefore, the giant magneto-resistance effect can be utilized as an insulator having good performance, so that a device having the giant magneto-resistance effect can store electric energy.
Since the capacitors disclosed in Patent Documents 1, 2 and 3 have the magnetic sections formed by thin films, the increased capacitance could be attained by expanding the magnetic sections bi-dimensionally, in which case however there would be a difficulty in downsizing the devices.
A thin-film capacitor, which uses electric energy stored in a charge collector formed on an interface between an electrode material and a dielectric material, is a capacitor of large capacitance capable of charging and discharging the capacitance in the order of thousands of farads in a moment.
As for usage, it is utilizable in a wide range from a capacitor with a low capacitance for backing up memories and one with a medium capacitance for assisting the power of electric vehicles up to one with a large capacitance as an alternative for batteries storing electric power, such as a power supply of electric vehicles.
The withstand voltage of a unit capacitor (also referred to as a cell) of such a thin-film capacitor is determined by the withstand voltage of the dielectric material which is a component thereof, i.e. an insulator which separates electrons from holes, and is dependent on its substance, film thickness, uniformity and the like. For example, the withstand voltage of barium titanate-based dielectric material is approximately 200V.
Since there is the possibility that a thin-film capacitor is broken when a voltage higher than the withstand voltage of the dielectric material is applied, a plurality of unit capacitors used are connected in serial.
As previously described, when electric charge stored in a single or connected capacitors is discharged, it is rapidly discharged, and thus, if it is discharged as it is, the output of the thin-film capacitor shows the decreasing curve A plotted in FIG. 3.
In order to smooth the output, an inverter is used, whereas it becomes unable to perform conversion when the capacitor output decreases below a threshold value.
Accordingly, in order to raise the threshold value, a capacitor having its capacitance at least equal to or higher than the threshold value is additionally installed in hybrid to smooth the output, thereby making it possible to discharge electrical charge until the stored amount of the storage capacitor becomes zero.
Generally, as a solution for charging and discharging of a capacitor, there is one disclosed for example in Patent Document 1.
In case of discharging a thin-film capacitor, when the thin-film capacitor is directly connected to an inverter as shown in FIG. 1, there has occurred such a situation that conversion does not take place with current lower than a threshold for the inverter, resulting in discharging uselessly.
It is an object of the present invention to provide a thin-film capacitor device, which can be implemented into an inexpensive configuration, by which a thin-film capacitor, compact in size and large in capacitance and capable of producing large electric energy, causes an inverter to effectively perform smoothing conversion during discharging until the stored amount becomes zero.